It is. I only worked 2 Christmas periods in retail. First one was awful, second one a little better because I had more authority and less patience for people/didn't feel like I had to be as patient with people.
Seriously, people get like extra self-entitled around the Christmas period. 70% of customers are just awful to be around at this time. You always get a few customers who are just lovely though, and make you want to go out of your way to help them, which is nice.